Permanent magnet brushless motors are alternating-current machines having electronic commutators whose purpose is to energize the windings of the motor in the correct correlation with the induced electromotive forces, sometimes called EMF, so that the motor produces an optimal torque.
In conventional direct current motors, the number of motor phases is equal to the number of windings and all of the phases are in series and are short-circuited to one another. The voltage/speed ratio of the motor determines the total number of turns required in the motor. Each windings must have a number of turns equal to the total number of turns in the motor divided by half of the number of phases in series. Small direct-current motors (50-300 watts) may include from 6 to 20 phases. Direct-current brushless motors require at least one electronic power switch for each of the motor's phases. In order to minimize the cost and the complexity of the electronics necessary for the switching, the number of phases must be reduced. In a typical configuration, brushless motors have two or three phases. The performance of direct-current brushless motors with three phases is comparable to that of a traditional direct-current motor. In a direct-current brushless motor, the number of turns for each phase is determined by the voltage/speed ratio of the motor and is typically higher than the number of turns in a conventional motor, since the phases are not in series. The higher number of turns results in a higher phase inductance and a correspondingly higher electrical time constant.
During switching, the electrical time constant determines the amount of time required for the phase current to rise from an initial level to the desired value. The torque produced by the phase of the motor is low during the switching period because of the low energization of the phase whilst the current rises to the desired value. If the electrical time constant is high, the phase current cannot rise to the desired steady state operating value during the period intended for the excitation of that phase at the operating speed. This causes reduced stedy-state torque performance at high speed. Moreover, the current-rise period generates an elevated current oscillation both in the motor and in the supply line. This oscillation of current produces undesirable electric noise and oscillation of the driving torque, which, in turn, produces undesirable acoustic noise.
A motor that is designed for unipolar or half-wave bridge drive has a time constant that is typically double that of a motor with the same performance designed for a bipolar or full-wave bridge drive. Therefore, the current oscillation in the unipolar motor/drive is a more serious problem than in the bipolar motor/drive. However, the unipolar configuration is typically less expensive than the bipolar configuration because it requires fewer switching devices. Comparing unipolar and bipolar configurations, a solution for the current oscillation problem is more advantageous for the unipolar motor/drive.
In the prior art, phase to phase switching techniques excite only one phase of the motor at a time, which results in undesirable current oscillation and torque variation during the switching transition periods. A conventional direct-current brushless motor is designed in such a way that the waveforms of the electromotive forces induced in the phases overlap each other. Ideally, these waveforms would overlap as illustrated in the first time chart of FIG. 1a. It can easily be seen in this time chart, that when the electromotive force induced in the first phase, E1, is at its maximum value, the electromotive force induced in the second phase, E2, is rising and is lower than the maximum value. Conversely, when the electromotive force E2 reaches the maximum value, the electromotive force E1 begins to decrease. Generally, for each time period, excluding transition, there is one, and only one, electromotive force at the maximum value.
In the second time chart of FIG. 1a, the corresponding currents IF1, IF2 are depicted for the two phases. At the time of transition, the current in the first phase IF1 drops to 0 with an almost vertical descending front. The current in the second phase IF2 rises to the maximum value with a more gradual path and the rate at which it rises to the maximum value is determined by the electric time constant discussed hereinbefore. As a result, the line current is not constant, but rather, as illustrated in the third time chart of FIG. 1a, the line current IL exhibits a "hole" due to the summing of the currents in phases IF1 and IF2. Consequently, the torque produced by the motor TQ, also exhibits a hole, as illustrated in the fourth time chart of FIG. 1a, and has a path that is substantially similar to the path of line current IL.
These oscillations, or holes, in the line current constitute a technical problem because of the resultant electrical noise and electromagnetic interference and the additional components that are required to filter this resultant noise and interference. Moreover, the oscillation of the line current generates a corresponding oscillation of the produced torque, that in turn generates undesirable acoustic noise and results in a reduced mean torque value that degrades motor performance.